Protective headgear and helmets have been used in a wide variety of applications and across a number of industries including sports, athletics, construction, mining, military defense, and others, to prevent damage to a user's head and brain. Contact injury to a user can be prevented or reduced by helmets that prevent hard objects or sharp objects from directly contacting the user's head. Non-contact injuries, such as brain injuries caused by linear or rotational accelerations of a user's head, can also be prevented or reduced by helmets that absorb, distribute, or otherwise manage energy of an impact. This may be accomplished using multiple layers of energy management material.
Conventional helmets having multiple energy management liners are able to reduce the rotational energy transferred to the head and brain by facilitating the rotation of the energy management liners against one another. Shaping the interface between energy management liners to have spherical symmetry would facilitate such a rotation. However, the consequences of such symmetry may include larger size, an undesirable length to width ratio, and/or decreased effectiveness due to insufficient energy management material.
Some conventional helmets, such as, for example, that disclosed in US Published application 20120060251 to Schimpf (hereinafter “Schimpf”), include a continuous interface surface between an inner liner and the outer liner. However, conventional helmet designs configured in this way are conventionally manufactured for football helmets, and are not suitable for conventional bicycle helmets where a large portion of the helmet is required to have air flow openings and gaps extending from the innermost area of the helmet through all energy management liners.
Furthermore, some conventional helmets, including some embodiments disclosed in Schimpf, employ a continuous surface interrupted by a recess in the outer liner that a projection from the inner liner extends into. Some conventional helmets employ structures or objects that bridge energy liners that must break or deform for the liners to rotate against each other. Such a method of energy absorption is disadvantageous; while the energy is absorbed by the failure or deformation of the projections, it happens over a short period of time, thus doing little to attenuate the rotational accelerations experienced by the user's head and brain.